There are many different types of data networks, of which Ethernet is perhaps the best known. Some data networks have resource reservation schemes. One such network is HomePNA (Home Phoneline Network Alliance) v3.1 which is designed to work over existing telephone lines to create a home/small office network. U.S. patent application Ser. No. 11/000,524, filed Dec. 1, 2004 and assigned to the common assignee of the present invention, describes generally how to extend the HomePNA v3.1 standard to operate over a hybrid network of telephone and coax lines.
HPNA v3.1 and other such resource reservation networks have a scheduler, described hereinbelow, to guarantee media resources to network devices, to prevent collision between multiple network devices using the same line and to ensure quality of service. In coax networks, preventive collision detection limits the dynamic range of the network devices, which may impose physical limitations on the size of the network, so it is preferable to use collision avoidance methods for media access in coax networks.
Such a collision avoidance method is disclosed in U.S. patent application Ser. No. 11/218,708 entitled ‘Collision Avoidance Media Access Method for Shared Networks’, filed Sep. 6, 2005 and assigned to the common assignee of the present invention. This application is incorporated herein by reference. The collision avoidance/carrier sensing media access (CA/CSMA) method disclosed in the application employs a media access plan (MAP) having sub-burst slots. Each sub-burst slot has a shorter duration than a minimal transmission burst duration (e.g., 8-32 μsecs), is associated with a particular one or group of network participants, and represents an opportunity for the initiation of a data transmission by its associated network participants.
The MAP for a transmission cycle dictates a schedule of sub-burst slots, wherein numbered sub-burst slots are scheduled in a particular order. FIG. 1A, reference to which is now made, shows an exemplary sub-burst slot schedule 10, in which five sub-burst slots numbered 0 through 4 are scheduled in sequential order. Sub-burst slot schedule 10 may also be seen as a grid of transmission opportunity start times. The start time STN for each sub-burst slot N is the moment at which the network participant associated with sub-burst slot N may begin to transmit.
In the initial grid of transmission opportunity start times (before any transmissions occur), the start time of each sub-burst slot N, STN, occurs after the sum of the durations of the sub-burst slots preceding sub-burst slot N. For example, as shown in FIG. 1A, the initial start times STi0, STi1, STi2, STi3, and STi4 of sub-burst slots 0-4 respectively, occur at (t=0), (t=d0), (t=d0+d1), (t=d0+d1+d2), and (t=d0+d1+d2+d3) respectively, where d0, d1, d2, and d3 are the durations of sub-burst slots 0-4 respectively.
The principal advantage of sub-burst slots over regular sized time slots is that when a network participant does not use its transmission opportunity, minimal time is wasted before the opportunity to transmit is passed to the next network participant in the queue. On the other hand, when a network participant opts to transmit when its turn arrives, the allowable transmission duration is not limited by the short duration of the sub-burst slot. Rather, the sub-burst slot expands to encompass the required transmission burst duration. Accordingly, the start times of the succeeding sub-burst slots are delayed by an amount of time equal to the portion of the transmission duration which exceeds the original sub-burst slot duration. In effect, the entire grid of transmission opportunity start times shifts by this amount.
For example, as shown in FIG. 1B, reference to which is now made, timing diagram 15 for an exemplary transmission cycle operating in accordance with sub-burst slot schedule 10 shows how a transmission during sub-burst slot ‘1’ alters the initial grid of transmission opportunity start times for the sub-burst slots following sub-burst slot ‘1’. As shown in FIG. 1B, start times STb2, STb3 and STb4 are incremented by x, the portion of the transmission transmitted during sub-burst slot ‘1’ which exceeds the original sub-burst slot duration d1.
In a network employing the CA/CSMA method described hereinabove, all of the participating network nodes receive the MAP and extract from it their relative transmission opportunities. Then they employ physical carrier sensing (PCS) to monitor transmissions occurring over the network so that, subsequent to each transmission, they can synchronize to an updated transmission opportunities (TXOPs) schedule accounting for transmission-induced shifts in the sub-burst slot start time grid.
Successful implementation of PCS is important for optimal operation of collision avoidance as described hereinabove. The carrier sensors in all of the network nodes must receive the same information regarding transmissions occurring over the network in order to guarantee synchronization of all nodes to the same timing and transmission opportunities schedule.